The present invention relates to a very efficient frequency plan for generating microwave signals for a wide-band, low noise radar system.
In one known radar system, the LO signal is generated by summing the microwave reference and frequency synthesizer signals, as shown in FIG. The function of the microwave reference is to generate wideband X-band signals with coarse frequency steps. Because of its high output frequency the microwave reference also sets the noise performance. The frequency synthesizer generates a narrow-band signal in a frequency region of less than 2 GHz with fine frequency steps (5 to 10 MHz) Thus, summing (mixing) of the two signals will generate a wideband microwave signal with fine frequency steps. The microwave reference is the most critical component in the wideband, low noise signal generator because it sets the overall radar signal bandwidth and the frequency stability. The transmit drive (TD) signal is generated by summing a constant frequency offset (TD Offset) to the local oscillator receiver (LO). For a conventional radar system with less than 10% bandwidth, the approach of FIG. 1 can provide excellent performance.
In a very wideband application, the approach of FIG. 1 is very hardware intensive (i.e., requires duplication of hardware), and spurious signal generation due to the TD Offset is very difficult to control. The Fault Tolerant Wideband Signal Generator frequency plan of U.S. Pat. No. 5,053,722, Kuo et al. and assigned to a common assignee with the present application, is useful for a radar system that requires very wide frequency bandwidth (e.g., 45%). U.S. Pat. No. 5,053,722 is incorporated herein in its entirety by this reference. In the approach of U.S. Pat. No. 5,053,722, the TD Offset is eliminated, thus reducing hardware and eliminating TD Offset spurious signal generation. The microwave reference is divided into high and low frequency bands such that the output frequencies for the two microwave references are separated by the TD Offset frequency. The outputs of the two microwave references are summed with the frequency synthesizer signal to produce the high band and the low band signals. By routing the TD signal from one band and the LO signal from another band, and interchanging bands by means of switching, it is possible to generate both TD and LO signals from a TD frequency band. In its most efficient form, the total bandwidth required from the two microwave references is approximately equal to the TD signal bandwidth minus twice the bandwidth of the frequency synthesizer.
It is therefore an object of the present invention to provide a wideband microwave signal generator wherein the required bandwidth of the microwave reference is substantially reduced, thereby reducing circuit complexity and improving the noise performance of the signal generator.